Electrolytic process of making organic dithionates



3,472,747 ELECTROLYTIC PROCESS F MAKING ORGANIC DITHIONATES George Smith, Richmond, Caliii, assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware No Drawing. Filed Aug. 13, 1965, Ser. No. 479,618 Int. Cl. Billk 1/00 US. Cl. 204-72 Claims ABSTRACT OF THE DISCLOSURE Organic thiocyanates are produced by electrolytic addition of alkali thiocyanate salts to an olefinically unsaturated organic reactant in liquid phase solution at controlled anode potential.

This invention relates to an improved method for the production of certain organic dithiocyanates.

Methods are known in the art for the production of organic dithiocyanates by the addition of two thiocyanate moieties to the carbon-carbon double bond of an olefin. In general, such methods involve the oxidation of a thiocyanate salt with an inorganic oxidizing agent, customarily elemental bromine, in the presence of the olefin the dithiocyanation of which is desired. Such methods are economically unattractive because of the high cost of the oxidizing agent and the formation of substantial quantities of by-products, e.g., organic bromides or dibromides, through reaction of the oxidizing agent with the olefin reactant.

It is an object of the present invention to provide an improved process for the production'of organic dithiocyanates. More particularly it is an object of the present invention to provide an electrochemical process for the dithiocyanation of organic unsaturates.

It has now been found that these objects are accomplished by the process of electrolyzing a solution of an inorganic thiocyanate salt in the presence of an organic unsaturate, which electrolysis is conducted at a controlled anode potential. Although the mechanism of the process is not known with certainty and it is not desired to be bound by any particular theory, it is considered probable that the electrochemical process results in the generation of thiocyanate radicals which subsequently undergo addition to the unsaturated moiety of the organic reactant.

The thiocyanate salt employed as a reactant in the process of the invention is a soluble inorganic thiocyanate, preferably an alkali thiocyanate, which term as employed herein is considered to be generic to the thiocyanates wherein the cation is that of a Group I-A metal, e.g., lithium, sodium, potassium, rubidium and cesium, as well as the thiocyanate salt wherein the cation is the ammonium ion, i.e., the NH ion, which because of the striking similarity in reaction character thereof is frequently classified as similar to the cations of the Group I-A metals. Preferred alkali thiocyanate salts are those wherein the cation is ammonium or sodium, and best results are obtained when ammonium thiocyanate is employed as United States Patent 0 3,472,747 Patented Oct. 14, 1969 the source of thiocyanate moieties in the process of the invention.

The organic reactant of the present process comprises an organic molecule incorporating within the molecular structure thereof a non-activated unsaturated system. By the term unsaturated system as employed herein is meant a continuous system of carbon atoms, the connecting linkages of which include at least one carbon-carbon double bond, which system functions chemically in the manner of a carbon-carbon double bond. In one modification, the unsaturated system is an ethylenic linkage per se, i.e., a non-aromatic carbon-carbon double bond, to which addition of thiocyanate moieties is effected in a 1,2- manner. In an alternate modification, the unsaturated system comprises two carbon-carbon double bonds which are conjugated and in this modification thiocyanate addition occurs in a 1,4-manner. The organic reactant therefore incorporates at least one and preferably only one unsaturated system which is an ethylenic linkage or is a system of two conjugated carbon-carbon double bonds. The unsaturated system is herein termed non-activated in that the double bond(s) of the unsaturated system is (are) not conjugated with non-carbon-carbon unsaturation, i.e., an unsaturated linkage one member of which is other than carbon. Thus, reactants wherein the unsaturated system is conjugated with moieties such as carbonyl, cyano, carboalkoxy or the like are not suitably utilized in the present process. The preferred organic reactants are reactants of a single non-activated unsaturated system as the only nonaromatic carbon-carbon unsaturation present within the molecule, and are reactants of up to 20 carbon atoms and of no atoms other than carbon, hydrogen and oxygen. The organic reactant is acyclic, carbocyclic or heterocyclic and incorporates any oxygen atoms present within the molecule in functional groups such as hydroxyl, oxy, carboxy, acyl and carbohydrocarbyloxy.

Illustrative of suitable acyclic organic reactants are ethylene, butene, pentene, decene, tetradecene, pentadecene, styrene, p-ethylstyrene, stilbene, butadiene, isoprene, 1,3-hexadiene, allyl alcohol, crotyl alcohol, methyl allyl ether, 3-butenyl acetate, oleic acid and 6-octenyl butyrate. Exemplary carbocyclic organic reactants include cyclohexene, 1,3 cyclohexadiene, cyclopentadiene, 4- propylcyclohexene, 1,4,5,6-tetrahydrobenzyl alcohol, cyclohexene-4-carboxylic acid, A -octahydronaphthalene, 1,4 dihydronaphthalene, cyclopentene, 3-hydroxycyclohexene, cyclooctene and 1,3-cyclooctadiene; and heterocyclic organic reactants include 3,4-dihydro-2H-pyran-2- methanol, A -butenolide and A -pentenolide. In general, acyclic organic reactants of from 4 to 16 carbon atoms are preferred over analogous carbocyclic or heterocyclic reactants, especially acyclic hydrocarbon organic reactants of this class.

The molar ratio of organic reactant to alkali thiocyanate salt is not critical. In general, molar ratios of from about 5:1 to about 1:5 are satisfactory, with molar ratios of from about 3:1 to about 1:3 being preferred.

The process of the invention is conducted in homogeneous liquid-phase solution in an inert reaction solvent. Although a variety of solvents which are liquid at reaction temperature, are capable of dissolving the reactants and products of the process of the invention and are inert thereto are suitably employed, preferred solvents are bydroxylic solvents, particularly those hydroxylic solvents of up to 6 carbon atoms represented by the formula wherein R is hydrogen, alkyl or alkanoyl. Thus, the preferred reaction solvents comprise water; alkanols of up to 6 carbon atoms such as methanol, ethanol, isopropanol, tert-butanol and n-hexanol; and alkanoic acids of up to 6 carbon atoms such as acetic acid, propanoic acid, and butyric acid; as well as mixtures thereof. Preferred solvents are the alkanols optionally employed in conjunction with equivalent or lesser volume amounts of water or alkanoic acid. Particularly suitable are ethanolic solvents of up to about 50% by volume water.

It has been found that control of the pH of the electrolyte solution, defined as the negative logarithm of the molar hydrogen ion concentration thereof, is of some importance in obtaining maximum yields of product. Although the process is operable when the electrolyte solution is basic, i.e., has a pH greater than 7, or when the solution is highly acidic, e.g., at a pH below about 1, best results are obtained when the electrolyte solution has a pH of from about 2 to about 7, preferably from about 4 to about 6. The method of controlling the pH of the solution is not critical. In one modification, the presence of alkanoic acid or other acidic components within the reaction medium serves to provide the desired degree of acidity. In alternate modifications, the pH of the solution is controlled by the addition to the reaction medium of other acidic material, as by passing carbon dioxide into the reaction medium or by adding minor proportions of a mineral acid such as hydrochloric acid, sulfuric acid or phosphoric acid. In the modifications wherein the electrolyte solution is not previously of a desired pH, sufiicient acidic material is added to bring the pH of the electrolyte solution within the desired range.

In the electrolysis process, the electrodes are composed of any material which is capable of conducting the electric current and which is inert to the reaction mixture and the products thereof. Typical electrode materials include platinum, nickel, graphite, copper, tin and lead.

The electrolysis process of the invention comprises charging the electrolyte solution to an electrolysis cell and passing an electric current between electrodes contained therein. For purposes of control of the anode reference potential, at standard electrode, typically a silver/silver chloride electrode or a Saturated Calomel Electrode (S.C.E.) such as is described in Weissberger, Physical Methods, New York, Interscience Publishers, Inc., 1960, vol. I, Part IV, is introduced into the cell in the vicinity of the anode and connected to the source of direct electric current and to the anode in a conventional manner so that the reference potential of the anode can be determined. It has been found that the potential of the anode is of some criticality in the production of the dithiocyanate products. During the course of a batch-type electrolysis, as thiocyanate salt is removed from solution by reaction with the organic reactant, the potential of the anode must be increased if a constant current density is to be maintained. However, rather than alter the anode potential in order to maintain a constant current density, it has been found desirable to maintain a constant anode potential and to allow the current density to vary if required. The utilization of a constant anode potential serves to minimize the formation of undesirable by-products, particularly the polythiocyanogen, i.e., the (SCN),,, which forms on the anode at high anode potential, and thereby obtain higher yields of product and increased current efficiency. In the preferred modification of the process, however, thiocyanate salt is added either continuously or in increments to the cell to replace that consumed by reaction, and the current density as well as the anode potential remains substantially constant. Anode reference potentials of from about 0.5 volt to about 0.9 volt vs. the Saturated Calomel Electrode are satisfactory with best results being obtained when an anode reference potential of from about 0.6 volt to about 0.8 volt (vs. S.C.E.) is employed.

The electrolysis process is conducted at moderately low temperatures. Reaction temperatures that are too low result in a lessened solubility of the reactants in the reaction medium whereas reaction temperatures that are too high lead to extensive formation of polythiocyanogen on the surface of the anode. Temperatures of from about 10 C. to about 15 C. are generally satisfactory with reaction temperatures of from about 5 C. to about 10 C. being preferred. As the reaction process is conducted wholly in the liquid phase, the efiiciency thereof is not dependent upon utilization of any particular reaction pressure. Largely for reasons of convenience, reaction pressures that are substantially atmospheric are preferred, e.g., pressures from about 0.5 atmosphere to about 2 atmospheres.

Subsequent to the electrolysis process the product mixture is separated and the desired dithiocyanate product is recovered by conventional methods, e.g., selective extraction, fractional distillation, fractional crystallization or the like.

The products of the invention are dithiocyanate compounds illustratively produced by the addition of a thiocyanate moiety to each of the terminal carbon atoms of the unsaturated system. Thus, in the modification wherein the unsaturated system is a carbon-carbon double bond, 1,2-addition of thiocyanate moieties is observed and the resulting product is a vie-dithiocyano derivative of the organic reactant. Illustrative products of this type are l,2-dithiocyano-l-phenylethane produced from styrene, 2,3-dithiocyano-1-propanol produced from allyl alcohol, 1,2-dithiocyanocyclohexane produced by reaction of cyclohexene and other typical products such as 9,10-dithiocyanostearic acid, 1,2-dithiocyanopentane, methyl 2,3- dithiocyanopropyl ether and 1,2-dithiocyanotetradecane. Alternatively, in the modification wherein the unsaturated system comprises two conjugated carbon-carbon double bonds, 1,4-addition of dithiocyanate moieties is observed and the product incorporates two thiocyanate moieties in an a,6-relationship. Illustrative products of this type include 1,4-dithiocyano-2-butene produced from butadiene, 1,4-dithiocyano-2-hexene produced by reaction of 1,3- hexadiene, 1,4-dithiocyano-2-methyl-2-butene produced by reaction of isoprene and other typical products such as 1,4- dithiocyano-2-cyclohexene and 1,4-dithiocyano-2-decene.

The products of the application find utility in a number of applications, particularly as chemical intermediates. The dithiocyano compounds are converted by known methods to the corresponding dimercapto derivatives useful in the formation of thioethers, thioesters and the like or as curing agents in the formation of epoxy resins. The products are useful as biological chemicals, particularly insecticides and the vie-dithiocyano derivatives are cyclized to form useful 1,3,2-dithiazole products.

To further illustrate the improved process of the invention, the following examples are provided. It should be understood that the details thereof are not to be regarded as limitations, as they may be varied as will be understood by one skilled in this art.

EXAMPLE I The reactor employed in this and the subsequent examples Was a cell of 750 ml. capacity which was fitted with a cylindrical platinum anode of 211 cm. surface area and an annular copper cathode shielded by an alundrum cup and supported on a stirring rod by a Teflon spacer. A thermometer, reference electrode and mechanical stirrer were also contained in the cell which was additionally equipped with a Dry Ice condenser.

To the cell was added 425 ml. of chilled ethanol and 108 g. of butadiene was slowly added followed by 114 g. of ammonium thiocyanate in ml. of water. The cell was cooled in an ice-salt bath and an anode potential of 0.6 volt vs. silver/silver chloride was set on the anode. The cell was kept below C. and generally below C. for 17.5 hrs., during which time 11.0 ampere-hours had passed through the cell. The Dry-Ice condenser was replaced by a water-cooled condenser and the cell was cyclohexene conversion of 4.35%. The current efiiciency was 36.4%. The product had the following elemental analysis.

Analysis.-Calc.: C, percent wt., 48.5; S, percent wt., 32.32. Found: C, percent wt., 48.6; S, percent wt., 32.1.

. 5 heated to 70 C. to remove 94.2 g. of unreacted butadlene. The contents of the cell were added to 1.5 liter of water EXAMPLE Iv and the mixture was extracted four times with 100 ml. The procedure of Example III was followed employing portions of chloroform. The extract was dried over maga reaction mixture of 282 g. oleic acid, 95 g. of ammonium nesium sulfate and the solvent was evaporated to yield a thiocyanate, 500 ml. of ethanol and 100 ml. of water. A light yellow semi-solid. Recrystallization from ethanol potential of 0.6 volt vs. silver/silver chloride was utilized gave 9.4 g. of 1,4-dithiocyanobutene-2, M.P. 8182 C., for an 18.5 hour period. Upon workup, 27.0 g. of 9,10- which represented a 21.5% yield based on a butadiene dithiocyanostearic acid was obtained, M.P. 7779 C., conversion of 2.76%. The current efiiciency was 42.4%. which represented a 49.6% yield based upon a conver- The product had the following elemental analysis. sion of 6.8%. The current efliciency was 32.8%. The

Analysis-Cale: C, percent wt., 42.3; H, percent wt., elemental analysis of the product was as follows: 3.53; S, percent wt., 37.7. Found: C, percent wt., 42.4; Analysis-Cale: C, percent wt., 60.3; H, percent wt.,- H, percent wt., 3.7; S, percent wt., 35.7. 8.5; S, percent wt., 16.1. Found: C, percent wt., 60.6; H,

ercentwt. 8.6 S, ercentw. 15.8. EXAMPLE 11 P p By a procedure similar to that of Example I, a series of EXAMPLE v experiments was conducted wherein the dithiocyanation By a procedure similar to that of Example I, the of styrene was effected under varying conditions. The dithiocyanation of a variety of unsaturates was effected. results of this series are shown in Table I. The observed product was the vic-dithiocyano derivative TABLE I Current Wt. Thioeyanate eflicieney, Conversion, Yield, styrene, g. Water, ml. Ethanol, ml. salt, g. Acid Amp-hrs. percent percent percent Rel. volts 300 300 NE, 172 Cone. H01, 10n11 13.3 22.3 5.9 41.36 0.6 0 570 NH4,100 Cone. H01, 10m1..- 4.34 71.3 6.6 58.6 0.6 200 400 Na,40.5 C0nc.HzSO4,25ml 11.0 9.33 3.9 20.2 0.6 630 ml. acetic acid NH4, None 2. 41 92.1 4. 13 45. 28 0.64 0 570 NH4,100 Cone. H01, 10m1 5.6 53.14 5.56 36.0 0.3 75 475 NH4, 172 37.0 10.03 6.95 43.9 0.0 200 5.0 16.1 1.5 3.3 0.65 200 5.6 14.4 2.27 6.8 1.0

EXAMPLE III of the reactant in each case, with the exception of the The dithiocyanation of cyclohexene was effected by a procedure similar to that of Example I. In this experiexperiment employing isoprene wherein 1,4-addition was observed. The results of this series are shown in Table II.

TABLE II Olefin Thiocyanate Current conversion, Yield, Reference Unsaturate, g Water, m1. Ethanol, ml. salt, g. Amp-hrs. efiiciency percent percent volts Isoprene, 67 280 200 7. 5 16. 2 2.22 7. 5 8. 4 Tetrahydrobenzyl alcohol, 112. 250 250 30. 5 20. 4 22. 0 44. 5 0. 6 allyl alcohol, 116.-- 550 0 12.6 18. 1 2. 22 22. 4 0. 8 l-totradeeene, 106. 100 500 ll. 1 19. 6 4. 05 39. 3 0. 6 D1hydropyran-2H methanol, 1 500 0 11. 4 0. 8 l. 8 4. 45 0. 6 l-tridecene, 182 100 500 10. 7 14. 2 2. 8 30. 3 0. 6

rnent, the charge to the reactor consisted of 164 g. cyclohexene, 95 g. ammonium thiocyanate, 425 ml. of ethanol and 150 ml. Water and the potential employed was 0.6 volt vs. silver/ silver chloride. After 12.8 amp-hours had been passed during 16.75 hours reaction time, the greater proportion of the solvent and unreacted cyclohexene were flashed off to provide a yellow-orange semi-solid residue which was treated with 15 0 ml. of water and extracted with four 75 ml. portions of chloroform. The extract was dried over magnesium sulfate and the solvent was removed by distillation under reduced pressure to afford a dark yellow residual liquid which was treated with benzene-pentane to give 22.8 g. of a semi-solid. Recrystallization from ethyl acetate afforded 17.2 g. of 1,Z-dithiocyanocyclohexane, which represented a 41.2% yield of product based on a I claim as my invention:

1. The process for the electrolytic 1,2- or 1,4-addition of thiocyanate moieties to the single unsaturated system of an organic reactant to produce an organic dithiocyanate by electrolyzing at an anode reference potential of from about 0.5 volt to about 0.9 volt vs. saturated calomel electrode a homogeneous liquid-phase solution of (a) said organic reactant having up to 20 carbon atoms, having no atoms other than carbon, hydrogen and oxygen and having a single unsaturated system, said unsaturated system being not conjugated with non-carbon-carbon unsaturation and being carboncarbon double bond or two conjugated carbon-carbon double bonds, and

(b) alklai thiocyanate, in hydroxilic solvent, said 7 solution have a pH of from about 2 to about 7, at a temperature of from about 10 C. to about 15 C., and recovering the organic dithiocyanate product. 2. The process of claim 1 wherein the alkali thiocyanate is ammonium thiocyanate.

3. The process of claim 1 wherein the alkali thiocyanate is sodium thiocyanate.

4. The process of claim 1 wherein the solvent com prises alkanol of up to 6 carbon atoms and up to about 50% by volume of water.

5. The process of claim 4 wherein the organic reactant is acyclic hydrocarbon of from 4 to 16 carbon atoms.

6. The process of claim 5 wherein the anode reference potential is from about 0.6 volt to about 0.8 volt vs. saturated calomel electrode.

7. The process of claim 6 wherein the alkali thiocyanate is ammonium thiocyanate.

8. The process of claim 7 wherein the organic reactant is styrene.

9. The process of claim 7 wherein the organic reactant is butadiene.

10. The process of claim 7 wherein the organic reactant is l-tetradecene.

References Cited UNITED STATES PATENTS 8/1931 Helwig 20472 FOREIGN PATENTS 4/ 1964 France.

US. Cl. X.R. 

